IMPORTANCE: COVID-19 infection has been associated with acute kidney injury. However, its possible association with long-term kidney function is not well understood. OBJECTIVE: To investigate whether kidney function decline accelerated after COVID-19 compared with after other respiratory tract infections. DESIGN, SETTING, AND PARTICIPANTS: This cohort study used linked data from the Stockholm Creatinine Measurements (SCREAM) Project between February 1, 2018, and January 1, 2022, in Stockholm, Sweden. All hospitalized and nonhospitalized adults in the database with at least 1 estimated glomerular filtration rate (eGFR) measurement in the 2 years prior to a COVID-19 positive test result or pneumonia diagnosis were selected. Statistical analyses were conducted between June 2023 and October 2024. EXPOSURE: COVID-19 and pneumonia (including influenza). MAIN OUTCOMES AND MEASURES: Mean annual change in eGFR after COVID-19 and after pneumonia was calculated with a linear regression model. RESULTS: The COVID-19 cohort comprised 134 565 individuals (74 819 females [55.6%]; median [IQR] age, 51 [37-64] years). The pneumonia cohort consisted of 35 987 individuals (19 359 females [53.8%]; median [IQR] age, 71 [56-81] years). The median (IQR) baseline eGFR was 94 (79-107) mL/min/1.73m2 for the COVID-19 cohort and 79 (61-92) mL/min/1.73m2 for the pneumonia cohort. After adjustment for covariates, both infections demonstrated accelerated annual eGFR decline, with greater magnitude of decline after COVID-19 (3.4% [95% CI, 3.2%-3.5%] after COVID-19; 2.3% [95% CI, 2.1%-2.5%] after pneumonia). This decline was more severe among individuals hospitalized for COVID-19 (5.4%; 95% CI, 5.2%-5.6%) but remained similar among those hospitalized for pneumonia. CONCLUSIONS AND RELEVANCE: This cohort study found an association between COVID-19 and accelerated decline in kidney function, particularly after hospitalization, compared with pneumonia. People who were hospitalized for COVID-19 should receive closer monitoring of kidney function to ensure early diagnosis and optimized management of chronic kidney disease to effectively prevent complications and further decline.